Propeller pitch control system



Jan. 13, 1959 H, M, HORN 2,868,304

PROPELLER FITCH CONTROL SYSTEM Filed Oct. 1. 1953 I 2 Sheets-Sheet 1 NVENTOR' HENRY M HURN ATTORNEY Jan. v13, 1959 H. M. HRN 2,868,304

PROPELLER PITCH CONTROL SYSTEM Filed` 0G13. 1. 1955 2 Sheets-Sheet 2 //v vE/vToR HE/vny M Hon/v ATORNEV United States Patent-*O 2,868,304 PROPELLER PITCH CONTROL SYSTEM Henry M. Horn, Bloomfield, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Application October 1, 1953, Serial No. 383,544

1t) Claims. (Cl. 170--160.2)

This invention relates to propeller control mechanism and particularly to electrical control systems for reversing and feathering propellers.

An object of this invention is to provide control circuits in which the possibility of malfunctioning is reduced.

A further object is to provide a throttle actuated pitch reversing control switch which may be mounted on the throttle control quadrant.

Other and additional objects will be apparent from the following description and attached drawings in which:

Fig. 1 is a schematic diagram showing the propeller with its controls and a schematic wiring diagram for the controls.

Fig. 2 is a perspective partially broken away of the reversing7 switch.

Fig. 2a. is a perspective exploded detail view of the switches and actuating means.

Fig. 3 is a side elevation, with parts broken away, of the switch of Fig. 2, showing the switch in latched position.

Fig. 4 is an enlarged schematic diagram of the governor showing the details of the solenoid valve and the pressure cut-out switch as well as the governor oil passages.

In the control of hydraulically actuated controllable pitch propellers of the type shown generally in Patent 'No. 2,663,373 for Reversible Pitch Propeller, issued December 22, 1953, to Nelson R. Richmond, and Patent No. 2,643,077, for Propeller Control Mechanism, issued .Tune 23, 1953, to Erle Martin and Calvin D. McCarthy, it has been customary to provide a push-pull control for feathering `and unfeathering and a push-pull or toggle type switch for reversing and unreversing. These pro-r pellers, which are commonly known as engine-oil type controllable pitch propellers, utilize a governor having a solenoid actuatedvalve therein, called a decrease pitch valve which overrides the constant speed function of the governor and directs oil in the proper direction to reduce the propeller pitch to unfeather or reverse the propeller. This decrease pitch valve is controlled by the unfeathering switch and by the reversing switch. One of the problems involved in unfeathering is the possibility that the blade might unfeather completely to a low pitch position or in fact might be inadvertently reversed if the unfeathering switch were held out too long. One of the features of the present construction is to disable the decrease pitch solenoid and valve by means of a blade switch when in unfeathering the blades have reached some predetermined high pitch position, much higher than the position determined by the low pitch stop. This circuit will act to cycle the pitch about the high pitch point at which the blade switch disables the decreased pitch solenoid until the unfeathering switch is released to allow the governor to take over control.

When propellers of this type are being feathered, it

occasionally happens that 4a surge of oil pressure will inadvertently actuate the Vcut-out switch and stop the feathering action before the propeller isfully feathered thus returning the propeller togovernor control. In

Patented Jan. 13, 1959 order to avoid this inadvertent malfunctioning of the feathering circuit a control ground connection is made through a blade switch as well as through the pressure cut-out switch so that the feathering action is assured until the blade is almost feathered at which time the ground through the blade switch is broken and the feathering is continued on the cut-out switch alone.

The action of reversing the propeller has been simplified by making the switch throttle-actuated and eliminating several of the relays forming a part of the electrical system. By using several switches mounted in a gang on the throttle control quadrant it has been possible to provide for manual actuation of the switches instead of the bulky relay actuation that has been commonly used. By using the small snap-acting switches it has been possible to mount the reversing switch directly on the throttle quadrant so that movement of the throttle in throttle closing direction past a detent will automatically actuate the reversing switch to reverse the propeller pitch and continued movement of the throttle in the same direction will open the engine throttle and apply more` engine power to the reversed propellers. One form of such a switch is shown in Bordelon Patent No. 2,579,172, issued December 18, 1951. In the present structure, however, provision-is made whereby the auxiliary pump relay is deenergized by the breaking of the relay circuit by a blade actuated switch in reversing and the decrease pitch valve is de-energized by breaking its circuit by a blade switch in unfeathering. In unreversing the throttle movement will manually close a switch to start the rauxiliary pump but another portion of the manually actuated switches is latched by a mechanical latch and held closed until a circuit is made by the blade switch which will energize the remainder of the reversing circuit and unlatch the mechanical latch and thus open all of the manually actuated reversing switches and disable the auxiliary pump and again return the propeller to governor control.

The attached drawings show one embodiment of the reversing switch and the feathering switch and their connections with the propeller. These drawings show a propeller and governor of the type shown in Patent Nos. 2,643,077 and 2,663,373 referred to above to which reference may be made for further details of the propeller and the governor. The present invention deals mainly with the control circuits for the propeller and governor.

In the drawings a propeller indicated generally at 10 (Fig. 1) has several blades, two of which are shown at 12 and 14 mounted in a hub 16 and operatively connected to an engine 18 .for rotation thereby. The propeller pitch changing mechanism is indicated schematically by a piston 20 mounted to reciprocate in a cylinder 22 to oscillate a gear 24 by means of a well-known pin and cam connection. Gear 24 meshes with similar gears (not shown) on the blade ends to impart pitch changing movements to the blades upon reciprocation of the piston Ztl. An engine driven hydraulic governor 26 which may receive its oil from the engine oil supply in a wellknown manner (not shown) boosts the pressure by a governor pump 114, 116 (Fig. 4) and directs this boosted engine oil through lines 28 and 30 to change the propeller pitch in one direction or the other to maintain constant speed in a well-known manner.

An auxiliary electric motor 32 drives an auxiliary pump 34 to supply high pressure iluid to the governor and as explained more fully in the above-identified applications, disables the governor action and directs fluid to one side of the piston 2i) to increase the propeller pitch. The pump 34 may draw its oil from any suitable source such as a separate oil sump 36.

Incorporated in the governor 26 is an electrically actuated decrease pitch valve 38 whichwhen energized will direct fluid, either governor pressure fluidl or auxiliary pump pressure fluid to the rear of the governor pump relief valve to increase the governor pump pressure and will also direct fluid to one side of the governor valve to thereby disable the governing action of the governor and move the governor valve to direct fluid from the governor to the side of the piston 20 which will decrease the propeller pitch.

Incorporated in the' governor is a pressure cut-out switch 40 which is normally closed but may be opened by an excessive pressure such as would be caused by a surge in the pitch changing lines connecting the governor with the pitch changing means 2l). This switch forms apart of the electrical system and its purpose will be explained more fully hereinafter. Mounted on each of two propeller blades are blade switches indicated schematically in the drawings as No. l blade switch and No. 2 blade switch. These switches comprise insulating segments Zand 44 on No. l blade switch and 46 and 48 on No. 2' blade switch. A brush 50 is arranged to alternately contact blade 1d which is grounded and the insulated sections d2 and 44 as the blade rotates under the brush. Brush 52 is mounted to alternately contact blade 12 which is grounded and insulated segments 46 and 4t; as the blade rotates under the brush. These two switches are utilized to make the break circuits in controlling the propeller pitch in a manner which will be Y more fully explained hereinafter.

rThe propeller is hydraulically actuated through the hydraulic governor', but the controls for the propeller are electrical and comprise generally a feather-unfeather switch 5d and a reversing control switch 56. Switch 54 when pushed to the left, as seen in Fig. 1, for feathering will connect contacts 58, 6d and 62` through the switch bar 6d to thereby connect a source of power such as a battery 6o with the switch hold-in coil ed and the auxiliary pump relay 76. Connected for movement with the bar dit of switch 54 is a bar 72 connecting contacts 74 and 76. Heldin coil 68 is grounded through pressure switch lll and, by means of bar 72 and contacts 7:5, is also grounded through No. 2 blade switch and propeller blade 12 so that after switch 54 has been pushed to make contacts to energize coil 68, the switch is held in by hold-in coil 63 to continue to make these contacts as long as coil 6d remains energized. it is necessary to break both the ground through the `pressure switch d@ and the ground through the No. 2 blade switch before coil 63 is cle-energized. No. 2 blade switch is set so that one of the insulated portions, say portion 48, of the blade switch breaks that ground connection several degrees, say f before the fcathering position. The final ground is broken when the propeller has been turned to its extreme feathering limit or against a mechanical stop, not shown, so as to create a pressure surge in the pitch increasing line which will actuate the pressure cut-out switch lill. When coil 68 is thus cle-energized, the feather-unfeather switch 54 returns to a central -of position by spring means (not shown). The return of the switch 54 to its central position will disconnect contact di* from the power source and :ie-energize auxiliary pump relay '7b and stop the auxiliary pump and motor. The propeller will then remain in the feathered position until unfeathering is initiated by pulling switch 54 to the right, as seen in Fig. 1, so that bar ed will connect contacts 7S, 8d, 82 and S4 and thus connect the source of power 66 with the auxiliary pump relay '711 and with a relay 86 which is grounded through No. 1 blade switch. Relay Se, which Imay be referred to as A relay, will, when energized, close switch SS onto contact 90 to thereby energize decrease pitch valve solenoid 38 which will move valve plunger 92 to connect the output of governor pump 114, 116 and the output of auxiliary pump 3d with line 236 leading to the area above the piston 216 formed on the governor valve stem 141%. Auxiliary pressure is led through line 94 t-o the rear ofv piston 96' tomove valve 98'so-as to connect line 94 with the underside of piston 216 mounted ou the governor valve stem and thus also supply auxiliary pressure to the underside of piston 216. Opening of decrease pitch valve 92 will also permit governor oil to be led by the way of check valve 16u to the rear side of piston 96 for a purpose to be explained hereinafter.

It is, of course, apparent that upon initiating unfeathering the governor is not rotating and there will be no pressure from the governor pump. It should be noted that auxiliary pressure fluid in line 94 is led by means off shuttle valve 102 to the rear of pump relief valve 148 to thereby increase the governor pump pressure when the governor pump does start to operate, which pressure will then be governed entirely by high pressure relief valve 104. lt will also be appreciated that, when the decrease pitch valve plunger 92 is moved by solenoid 38, the governor pump pressure may be led through check valve 10d to line 94 and hence to the rear of relief valve 148 to thereby increase the pressure of the governor pump by its own output. After the propeller begins to windmill, the unfeathering switch 54 may be released to allow it to again center and become inactive which will stop the auxiliary pump and close the decrease pitch valve 92 which will connect both sides of piston 216 with drain and will return the control to the governor.

lu the event the unfeathering switch 54 is held out, the ground through the A relay is broken by the No. l blade switch by one of the insulated portions, say portion 44, coming under the brush 5d. This breaking of the ground in unfeathering will occur near the high pitch limit. Ungrounding of the A relay will open switch 88 and de-energize the decrease pitch valve but the unfeathering switch still being held out will continue to energize the auxiliary pump relay 70 and supply auxiliary pump pressure to the governor which will supply high pressure oil to the underside of piston 216 forcing the governor valve upward to thereby direct oil to the pitch increasing side of pitch changing piston 20. The propeller will therefore tend to increase pitch which will move insulated portion 44 out from under brush 50 to again complete the ground for the A relay thus again closing switch ES and energizing the decrease pitch valve solenoid to again move the propeller blades in a decrease pitch direction. The blades will thus cycle about this high pitch position as long as the unfeathering switch is held out. As indicated above, release yof the unfeathering switch will return the propeller to governor control.

A reversing control switch 56 is provided to reverse and unreverse the propeller. This switch is preferably mounted directly on the throttle quadrant in the pilots cockpit and as schematically shown in Fig. 1, the throttle control 106 which is connected to a throttle, shown as arm 103, carries a cam 110 mounted for movement with 'the throttle 106. Cam 11G has a lobe 112 which, when the throttle is moved past a detent, not shown, in a throttle closing direction past throttle closed position, will contact `a roller 113 mounted on a spring lever 124) (Fig. 2). A plunger 122 is mounted under lever 12u in a position to be reciprocated by movement of the lever 120. A series of snap acting switches, four of which are shown in Figs. 2 and 2a and indicated-at E1, E2, S1 and S2, are mounted in a framework in the reversing` control switch 56. A plurality of levers 121 and 123 hinged at 124 are mounted for oscillation in front of the several switches. An anvil 126 is mounted on the end of plunger 122 so as to contact the several levers 121 and 123. Reciprocating movement of the plunger 122 induced by contact of roll 118 with the cam lobe 112 will therefore oscillate levers 121 and 123 about their pivot 12d to contact the switch buttons, one of which is shown at 125, to actuate the several switches E1, E2, S1 and S2.

The reversing control switch is encased in a box 127 which may be mounted by studs 1278 directly on the control quadrant, not shown. In addition to the switches E1', E2, S1 andl S21 which are enclosed in the box 127',

there is a relay, which may be referred to as the F relay, comprising a solenoid 130` and a switch 132. Also eri- `closed in the box 127 is a latch and means for electrically actuating the same. The latch comprises a lever 134 pulled `by spring 136 around pivot 137 away from the magnet pole piece 138. The lever 134 is mountedso as to swing across they path of an extension 141 of the lever 121 as shown in Fig. 3 as it moves about pivot 124. When the lever 121 is moved by the plunger 122 so as to close the `switches E1 and E2 the lever 134 will be swung by spring 136 in back of extension 141 so as to hold leverV 121 in position with the switches closed. Energization of coil 142 of the latch magnet will draw lever 134 to the right, as seen in Fig. 3, to thus unlatch lever 121 and allow the springs 143 to open switches E1 and E2.

As shown in the schematic diagram in Fig. l, movement of throttle lever 106 so as to bring cam lobe 112 into contact with roller 11S will mechanically close switches S1,l S2, E1, and E2. Closing switch E2 will supply power from the battery 66 to one side of magnet coil 142 and to switch lever E1. Closing switch lever E1 will supply power to one side of the F relay coil land to switch lever S2. Closing switch S2 will actuate the decrease pitch valve solenoid. Actuating switch S1 will,complete the circuit through the F relay coil to the ground connection through the No. 2 blade switch. Energization of the F relay coil 130 will close the switch 132 to thus supply power to auxiliary pump relay 70 and start the auxiliary pump. i

With the decrease pitch valve energized the propeller will decrease its pitch rapidly if the auxiliary pump is operating and less rapidly if the governor pump is the sole source of power. It should be noted that the decrease pitch valve is controlled by a mechanically actuated switch so that in the event of failure of the F relay or the auxiliary pump relay the propeller may be reversed through the operation of mechanically actuated switches alone by means of the decrease pitch valve and the governor pump alone. In this event fluid is led from the governor pump 114, 116 through valve 92 and check valve 100 to the rear of piston 96 and hence through passage 94 and shuttle valve 1112 to the rear of relief valve 148. This will increase the pressure delivered by pump 114, 116 sufficiently to overcome the usual low pitch stop and reverse the propeller.

When the propeller reaches its reverse pitch position `an insulated portion of the No. 2 blade switch, say section 46, will come under brush 52v to therebybreak the ground of the F relay and thus de-energize auxiliary pump relay 70 and stop the pump motor. The S2 switch, however, remains closed as long as the control lever is in the reverse position, to thereby keep the decrease pitch valve energized and thus hold the propeller in reverse pitch under the influence of governor oil pressure alone.

When it is desired to `unreverse the propeller control lever 106 is actuated to move cam lobe 112 away from roller 118 thus allowing lever 123 to move about its pivot and switches S1 and S2 to open. Switches E1 and E2 now constitute unreverse switches and are however held closed by the latch 134. Switch S1 in its open position is directly I,grounded which with E1 closed will complete the circuit through the F relay to start the auxiliary pump and motor. Opening of the S2 switch will however de-energize the decrease pitch valve so that supplying auxiliary pump pressure to the governor will now through the action of piston 96 and valve 98 cause the governor to direct oil to the propeller so as to move the propeller in a pitch increasing direction i. e., toward a high pitch position so that theauxiliary pump, piston 96 and valve 98 will act as pitch unreversing means. The propeller will continue to move toward the high pitch position until an insulated portion of No. l blade switch, say portion 42, moves out from under brush 50 thus allowing brush 50 to complete the ground through magnet coil 142Vin the reversing control switch and thus pull latch 134 to the right as viewed in Fig. 3 to thus unlatch lever 121 allowing switches E1 and E2 to open under the influence of springs 143. Opening of switch E2 will remove the source of power from the F relay thus returning the propeller to governor control. The insulated portion 42 is positioned to move out from under the brush 50 about six degrees above the normal low pitch position of the propeller. The propeller will thus continue to operate under governor control in the usual manner until either the feather button is pushed or the throttle moved to the reverse position.

Failures in the control circuit, if failure occurs, are usually in only one element such as a switch sticking or a coil shorting or opening or a relay otherwise failing. The above circuit is constructed so that failure of any one element will not have a serious adverse effect.

inadvertent reversing, such as reversing from a governing position, or when attempting to feather is considered most serious and failure to feather when required might be considered the next most serious malfunction. No single failure in either the reversing or feathering circuit will cause reversing while the propeller is under governor control. The manual operation of feathering as initiated by pushing the feathering button cannot be adversely affected by a single failure within the reversing control. Most single failures will make themselves obvious, usuallyV during the reversing and unreversing operations which functions are usually performed on the ground where.- the malfunctioning can be corrected.

From the above-description it is apparent that I have provided a propeller control circuit in which in additionV to the above mentioned safety features a safety feature has been provided in the feathering circuit which will cause.` the propeller to continue to the feathered. position evenv if the pressure actuated switch should inadvertently be temporarily opened by a pressure surge while feathering.. I have also provided a positive means i. e., a blade switch, for disabling the auxiliary pressure source at the reverse' position to prevent excessive pressures in the reverse posi-- tion. I have also provided a single blade switch for over-- riding the pressure control switch during the feathering operation and for terminating pump action at reverse position and a single blade switch for breaking both the decrease pitch valve circuit during unfeathering and the pump circuit during unreverse movement. I have also provided a compact and eicient reverse control switch for use in the control system, including mechanically actuated switches for accomplishing the reversing ot' the propeller.

While one embodiment of the invention has been shown and has been described and illustrated inthe accompanying drawings it will be understood that this particular embodiment of the invention is shown for purposes of illustration only and that various changes in the construction and operation of the mechanism may be made without departing from the spirit and the scope of the invention as defined in the appended claims.

What it is desired to secure by Letters Patent is:

l. In a controllable pitch propeller control system, in combination, a relay, a source of power, an unfeather switch connecting said relay with said source of power, a governor, pitch decreasing means for disabling'said governor and directing tluid through said governor to said propeller to reduce the pitch thereof, means connecting said pitch decreasing means with said relay for energizing said pitch decreasing means upon closing of said relay, an unreversing switch for unreversing said propeller, a latch for said unreversing switch, an electrically actuated unlatching means for unlatching said latch, pitch unreversing means connected with said propeller, said unreversing switch connecting said pitch unreversing means and said unlatching means with said source of power, a blade switch arranged in series with the coil of said relay and said unfeather switch and with said unlatching means and `said unreversing switch, brush 7 means forming a part of said blade switch and both series. circuits, said blade switch completing the yunfeather circuit from feathered position to a preselected high pitch position and then breaking said unfeather circuit and deenergizing said relay and disabling said pitch decreasing means, said blade switch breaking the circuit through said unlatching means from reverse to a predetermined low pitch position and then completing the circuit through said unlatching means to unlatch said unreversing switch i and disablesaid unreversing means.

2. In a control system fora .hydraulically actuated controllable pitch propeller having a reversed position and a feathered position, an auxiliary source of fluid pressure for reversing and for feathering said propeller, a

' source of energy, a reverse switch, including a relay, connecting said auxiliary source with said energy source for energizing said auxiliary source, a feather switch connecting said auxiliary source with said energy source for energizing said auxiliary source, a hold in coil for said feather switch, connected with said energy source by said feather switch, a coil for said relay connected with said energy source by said reverse switch, a blade switch connected with said feather switch hold in coil by said feather switch and with said relay coil by said reverse switch and completing the energy `circuit through both of said coils, a switch actuated by huid pressure in said propeller arranged in parallel with said blade switch also completing the circuit through said feather switch hold in coil, said blade switch breaking the circuit through .said relay coil at the reversed position to de-energize said auxiliary source and breaking one path of the circuit through said feather switch hold in coil adjacent feathered position, said pressure actuated switch breaking the other path of the circuit through said feather switch at feather position to de-energize said auxiliary source.

3. In an unfeathering system for a hydraulically actuated controllable pitch propeller having a governor and an electrically actuated valve for disabling said governor and decreasing the propeller pitch and an electrically driven auxiliary pump for disabling said governor and increasing the propeller pitch, a relay having a switch and a coil, an un'feathering switch connecting said pump, said relay switch and said relay coil with a source of energy, a blade actuated switch completing the circuit between said coil and said source, said relay switch connected with, and when energized actuating, said decrease pitch valve, said blade actuated switch opening to deenergize said relay coil and stop pitch decreasing action at a preselected high pitch position, said auxiliary pump then acting to increase the propeller pitch and again close the blade switch to cycle said propeller about said high pitch position.

4. In a feathering syste-1n for a hydraulically actuated controllable pitch propeller, a feathering switch for moving the propeller pitch toward feathered position, a hold in coil for said switch, said featheringswitch connecting one side of-said coil with a source of energy, a switch actuated by the pressure of fluid in said propeller and a v switch actuated by movement of the blades of said propeller arranged in parallel and completing the circuit between the other side of said coil and said energy source, said blade actuated switch breaking its portion of the circuit adjacent the feathered position and thc fluid actuated switch breaking its portion of the circuit at the feathered position.

5. A reversing control for a reversible hydraulically actuated controllable pitch propeller having a normal range, a reverse pitch position, an electrically driven source of auxiliary uid pressure for moving the propeller in an unreversing pitch direction and electrically actuated means for directing said tluid pressure to move the propeller in a reverse pitch direction comprisingin combination, manually actuated switching means movable in one direction to energize both said electrically actuated directing means and said electrically driven source, and movable in the other direction to de-energize said electrically actuated directing means and energize said electrically driven source, a iirst blade actuated switch connected with said electrically driven source for disabling said source when the propeller reaches reverse pitch position, and a second blade actuated switch connected with said electrically driven source for disabling said source at a preselected position in said unreversing movement.

6. A device as claimed in claim 5 in which the first blade actuated switch breaks a circuit to disable said source, and the second blade actuated switch makes a circuit to disable said source.

7. A reversing switch comprising, a plurality of aligned switches spring urged in one direction, a pair of flat plates hinged at one end adjacent said switches and movable about their hinges to operate said switches against the action of said springs, atleast onev of said plates operating more than one of said switches, a reciprocable plunger extending transversely across both hinged plates and movable to simultaneously actuate both plates and their associated switches, a latch arranged to move transversely of said plates, resilient means for moving said latch back of one of said plates after it has been moved by said plunger, means for returning said plunger and the other plate to inactive position while the one plate is latched and electrically actuated means to move said latch from the path of said one plate.

8. A reversing control for a reversible hydraulicallyactuated controllable-pitch propeller having a normal range, a reverse pitch position, an electrically driven source of auxiliary iluid pressure for moving the propeller in an unreversing pitch direction and electrically actuated means for directing said iluid pressure to move the propeller in a reverse pitch direction, comprising in combination, manually actuated switching means comprising a plurality of aligned switches, a pair of hinged plates simultaneously manually movable in one direction to operate said switches to energize both said electrically actuated directing means and said electrically driven source, a rst blade actuated switch connected with said electrically driven source disabling said source when the propeller reaches reverse pitch position, one of said plates operating more than one of said aligned switches, the switches actuated by one plate being movable in the opposite direction by springs to de-energize said electrically actuated directing means and energize said electrically driven source, a latch for restraining one plate and its associated switches from movement by springs in said opposite direction, electrically actuated means disabling said latch, a second blade actuated switch energizing said electrically actuated means to disable said latch at a preselected position in the unreversing movement of said propeller to disable said electrically driven source.

9. A reversing switch for a reversing control of a reversible hydraulically-actuated controllable-pitch propeller having a normal range, a reverse pitch position, and an electrically driven source of fluid pressure for changing the propeller pitch, comprising a plurality of aligned switches spring-urged in one direction, a pair of at plates hinged at one end adjacent said switches and movable about their hinges to operate said switches against the action of said springs, at least one of said plates operating more than one of said switches, a reciprocable plunger extending transversely across both hinge plates and movable to simultaneously actuate both plates and their associated switches and energize said source to move the propellers to reverse pitch-means to de-energize said source at reverse pitch, a latch arranged to move transversely of said plates, resilient means for moving said latch back of one of said plates after it has been moved by said plunger, means for returning said in said unreversing movement for energizing said elec- 5 trically actuated means.

10. In a hydraulically-actuated, electrically-controlled, controllable-pitch, reversing propeller, having a governor with an electrically actuated decrease pitch valve for disabling the governor and directing uid to said propeller 10 to reduce the propeller pitch, a first set of switches for feathering and unfeathering said propeller including a pair of switches connectable in series for activating said valve,

10 a second set of switches for reversing and unreversing said propeller including at least two switches connectable in series for activating said valve, whereby no single failure in the reversing or unfeathering circuit will cause inadvertent reversing.

References Cited in the file of this patent UNITED STATES PATENTS 1,288,738 Suren Dec. 24, 1918 2,288,406 Kimball June 30, 1942 2,663,373 Richmond Dec. 22, 1953 2,664,9604 Longfellow et al. Jan. 5, 1954 2,731,094 Hine Jan. 17, 1956 

